1. Field of the Invention
The present invention relates to a process for the heterogeneously catalyzed gas-phase oxidation of propane to acrolein and/or acrylic acid, in which a reaction gas starting mixture comprising propane, molecular oxygen and, if desired, inert gas is passed at from 300 to 500xc2x0 C. over a fixed-bed catalyst.
2. Discussion of the Background
Acrolein and acrylic acid are important intermediates which are employed, for example, for producing active compounds and polymers.
At present, by far the most widely employed process for the industrial preparation of acrolein and/or acrylic acid is the gas-phase catalytic oxidation of propene (for example EP-A 575897), with the propene being mostly produced as a by-product of ethylene production by steam cracking of naphtha. Since the other application areas for propene, e.g. the production of polypropylene, continue to expand, it would be advantageous to have an industrially usable, competitive process for preparing acrolein and/or acrylic acid which uses as raw material not propene but propane which, for example, occurs naturally in large amounts as a constituent of natural gas.
EP-A 117146 proposes preparing acrylic acid from propane by converting propane into a propylene-containing product stream by means of heterogeneous catalytic dehydrogenation in the absence of molecular oxygen in a first stage and, in subsequent oxidation stages, passing this product stream together with molecular oxygen over suitable oxidation catalysts so as to oxidize the propene present therein to acrolein and/or acrylic acid.
A disadvantage of this procedure is that it necessarily requires a plurality of reaction stages, with the individual reaction stages having to be carried out under different reaction conditions.
Furthermore, the abovementioned procedure has the disadvantage that the catalyst required for the nonoxidative dehydrogenation of the propane is relatively quickly deactivated as a result of carbon deposits and has to be regenerated. Since the dehydrogenation product mixture also contains hydrogen, CN-A 1105352 casts doubt on the possibility of passing the dehydrogenation product mixture on directly to a subsequent oxidation stage.
Both CN-A 1105352 and Y. Moro-oka in Proceedings of the 10th International Congress on Catalysis, Jul. 19-24, 1992, Budapest, Hungary, 1993, Elsevier Science Publishers B. V., pp. 1982 to 1986, recommend first converting propane partially into propene in a homogeneous oxidative dehydrogenation and converting this propene, without separating it off beforehand, into acrolein and/or acrylic acid in subsequent heterogeneously catalyzed oxidation stages. Disadvantages of this procedure are, on the one hand, that carbon is also formed in a homogeneous oxidative dehydrogenation of propane to propene and, on the other hand, that the selectivity of the formation of the desired product (acrolein and/or acrylic acid) is not satisfactory in such a procedure. Thus, in the examples in CN-A 1105352, the selectivity of propene formation achieved by homogeneous oxidative dehydrogenation is only xe2x89xa640% by volume and Moro-oka is also restricted to selectivities of acrolein formation of 64 mol %, based on propane reacted.
It has also been proposed that a heterogeneously catalyzed oxidative dehydrogenation of propane (which is not necessarily accompanied by carbon formation) be coupled with a subsequent heterogeneously catalyzed oxidation of the propene thus produced to give acrolein and/or acrylic acid (e.g. 210th ACS National Meeting, Chicago, Aug. 20-24, 1995 or WO 97/36849). However, further details regarding the type and manner of the coupling (in general, both reaction steps require reaction conditions which cannot be reconciled) were not given. CN-A 1105352 even advises decidedly against such a coupling, since, at reasonable propane conversions, achievable selectivities of propene formation in a heterogeneously catalyzed oxidative dehydrogenation do not exceed those in a homogeneous oxidative dehydrogenation.
Topics in Catalysis 3(1996), pp. 265-275, reports the heterogeneously catalyzed oxidative dehydrogenation of propane to propene over cobalt molybdate and magnesium molybdate. A disadvantage of the procedure of the abovementioned reference is that it is, presumably to ensure a satisfactory selectivity of propene formation, carried out in high dilution, i.e. the reaction gas starting mixture comprising propane and molecular oxygen further comprises up to 75% by volume of molecular nitrogen (inert gas). Such a high proportion of inert gas does not encourage coupling with a subsequent propene oxidation, since it reduces the space-time yields of acrolein and/or acrylic acid achievable in a single pass. Furthermore, such a proportion of nitrogen makes it more difficult to recirculate unreacted propane and/or propene after having separated off acrolein and/acrylic acid, should this be intended subsequent to the propene oxidation.
Journal of Catalysis 167 (1997), 550-559 likewise reports the heterogeneously catalyzed oxidative dehydrogenation of propane to propene or molybdates. A disadvantage of the procedure in this reference is that it likewise recommends the use of a reaction gas starting mixture whose proportion of molecular nitrogen is 70% by volume. Furthermore, the abovementioned reference proposes a dehydrogenation temperature of 560xc2x0 C. Such a high dehydrogenation temperature likewise does not suggest coupling to a downstream heterogeneously catalyzed propene oxidation, since it damages the multimetal oxide compositions customarily used for an oxidative conversion of propene into acrolein and/or acrylic acid.
Journal of Catalysis 167 (1997), 560-569 likewise recommends a dehydrogenation temperature of 560xc2x0 C. for a heterogeneously catalyzed oxidative dehydrogenation. Similarly, DE-A 19530454 similarly recommends temperatures above 500xc2x0 C. for a heterogeneously catalyzed oxidative dehydrogenation of propane to give propene.
Furthermore, experiments on a heterogeneously catalyzed direct oxidation of propane to acrolein and/or acrylic acid are reported in the literature (e.g. Proceedings, 210th ACS National Meeting, Chicago, Aug. 20-24, 1995, FR-A 2693384 and 3rd World Congress on Oxidation Catalysis, R. K. Grasselli, S. T. Oyama, A. M. Gaffney and J. E. Lyons (Editors), 1997 Elsevier Science B.V., pp. 375-382), although in these studies, too, either the reported selectivity of the acrolein and/or acrylic acid formation and/or the reported yield of acrolein and/or acrylic acid on a single pass are not satisfactory.
EP-B 608838 likewise relates to the heterogeneously catalyzed direct oxidation of propane to acrylic acid. However, a disadvantage of the method disclosed in EP-B 608838 is that the selectivities of the acrylic acid formation reported by way of example in this document cannot be reproduced. Thus, our attempts to repeat the work gave a vanishing selectivity for acrylic acid formation. Instead, formation of acrolein was found when repeating these examples, but the selectivity for the acrolein formation was only xe2x89xa630 mol %.
It is an object of the present invention to provide a process for the heterogeneously catalyzed gas-phase oxidation of propane to acrolein and/or acrylic acid, in which a reaction gas starting mixture comprising propane, molecular oxygen and, if desired, inert gas is passed at from 300 to 500xc2x0 C. over a fixed-bed catalyst, which process does not have the disadvantages of the methods described and/or recommended in the prior art.
We have found that this object is achieved by a process for the heterogeneously catalyzed gas-phase oxidation of propane to acrolein and/or acrylic acid, in which a reaction gas starting mixture comprising propane, molecular oxygen and, if desired, inert gas is passed at from 300 to 500xc2x0 C. over a fixed-bed catalyst which comprises two catalyst beds A and B arranged spatially in succession, with the proviso that the active composition of bed A is at least one multimetal oxide of the formula I
Ma1Mo1xe2x88x92bMb2Ox
where M1=Co, Ni, Mg, Zn, Mn and/or Cu, preferably Co, Ni and/or Mg, particularly preferably Co and/or Ni,
M2=W, V, Te, Nb, P, Cr, Fe, Sb, Ce, Sn and/or La, preferably Sn, W, P, Sb and/or Cr, particularly preferably W, Sn and/or Sb,
a=0.5-1,5, preferably 0.7-1.2, particularly preferably 0.9-1.0,
b=0-0.5, preferably  greater than 0-0.5 and particularly preferably 0.01-0.3, and
x=a number which is determined by the valence and amount of the elements different from oxygen in I,
and the active composition of bed B is at least one multimetal oxide of the formula II
Bia, Mob, X1c, X2d, X3e, X4f, X5g, X6h, Ox,xe2x80x83xe2x80x83(II),
where
X1=W, V and/or Te, preferably W and/or V
X2=alkaline earth metal, Co, Ni, Zn, Mn, Cu, Cd, Sn and/or Hg, preferably Co, Ni, Zn and/or Cu, particularly preferably Co, Ni and/or Zn,
X3=Fe, Cr and/or Ce, preferably Fe and/or Cr,
X4=P, As, Sb and/or B, preferably P and/or Sb,
X5=alkali metal, Tl and/or Sn, preferably K and/or Na,
X6=rare earth metal, Ti, Zr, Nb, Ta, Re, Ru, Rh, Ag, Au, Al, Ga, In, Si, Ge, Th and/or U, preferably Si, Zr, Al, Ag, Nb and/or Ti,
axe2x80x2=0.01-8, preferably 0.3-4 and particularly preferably 0.5-2,
bxe2x80x2=0.1-30, preferably 0.5 to 15 and particularly preferably 10-13,
cxe2x80x2=0-20, preferably 0.1 to 10 and particularly preferably 0.5-3,
dxe2x80x2=0-20, preferably 2-15 and particularly preferably 3-10,
exe2x80x2=0-20, preferably 0.5-10 and particularly preferably 1-7,
fxe2x80x2=0-6, preferably 0-1,
gxe2x80x2=0-4, preferably 0.01-1,
hxe2x80x2=0-15, preferably 1-15 and
xxe2x80x2=a number which is determined by the valence and amount of the elements different from oxygen in II,
wherein the reaction gas starting mixture comprises xe2x89xa750% by volume of propane, xe2x89xa715% by volume of O2 and from 0 to 35% by volume of inert gas and flows through the catalyst beds A and B in the order first A, then B.
Preferred multimetal oxides I are accordingly those of the formula Ixe2x80x2
[Co, Ni a./o. Mg]a Mo1xe2x88x92b [Sn, W, P, Sb a./o. Cr]bOxxe2x80x83xe2x80x83(Ixe2x80x2),
where a=0.5-1.5, preferably 0.7-1.2, particularly preferably 0.9-1.0,
b=0-0.5, preferably  greater than 0-0.5 and particularly preferably 0.01-0.3, and
x is a number which is determined by the valence and amount of the elements different from oxygen in Ixe2x80x2.
Particularly preferred multimetal oxides I are those of the formula Ixe2x80x3
[Co a./o. Ni]a Mo1+b [W,Sn a./o. Sb]bOxxe2x80x83xe2x80x83(Ixe2x80x3),
where a, b and x are as defined above.
Preferred multimetal oxides II are those of the formula IIxe2x80x2
Bia, Mob, Wc, [Co,Nia./o.Zn]d, Fee, [Pa./o.Sb]f, [Ka./o.Na]g, X6h, Ox,xe2x80x83xe2x80x83(IIxe2x80x2),
where X6 and the stoichiometric coefficients are as defined for formula II.
Particularly preferred multimetal oxides IIxe2x80x2 are those in which X6=Si, Zr, Al, Nb, Ag and/or Ti, among which preference is in turn given to those in which X6=Si.
It is also advantageous for exe2x80x2 to be 0.5-10, particularly when X6=Si.
The above applies particularly when the multimetal oxide compositions IIxe2x80x2 as described in EP-B 575897 are being prepared.
Particularly advantageous catalyst bed pairs A, B are the combinations Ixe2x80x2, IIxe2x80x2 and Ixe2x80x3, IIxe2x80x2. This applies particularly when X6=Si and exe2x80x2=0.5-10.
In the process of the present invention, the reaction gas starting mixture is advantageously passed over the fixed-bed catalyst comprising catalyst beds A and B at from 325 to 480 or 450xc2x0 C., preferably from 350 to 420xc2x0 C. and particularly preferably from 350 to 400xc2x0 C. Normally, the catalyst beds A, B have identical temperatures.
If the catalyst bed pair A, B used is a combination Ixe2x80x2, IIxe2x80x2 or Ixe2x80x3, IIxe2x80x2, the reaction temperature in both beds is advantageously from 350 to 420xc2x0 C., frequently from 350 to 400xc2x0 C.
Furthermore, the reaction gas starting mixture advantageously comprises xe2x89xa630% by volume, preferably xe2x89xa620% by volume and particularly preferably xe2x89xa610% by volume or xe2x89xa65% by volume, of inert gas. Of course, the reaction gas starting mixture can also contain no inert gas. In the present context, inert gases are gases which are reacted to an extent of xe2x89xa65 mol % on passing through the fixed-bed catalyst to be used according to the present invention. Possible inert gases are, for example, H2O, CO2, CO, N2 and/or noble gases.
Moreover, the reaction gas starting mixture advantageously comprises xe2x89xa760% by volume or xe2x89xa770% by volume or xe2x89xa780% by volume of propane. The propane content of the reaction gas starting mixture to be used according to the present invention is generally xe2x89xa685% by volume, frequently xe2x89xa683 or xe2x89xa682 or xe2x89xa681 or xe2x89xa680% by volume. The amount of molecular oxygen present in the reaction gas starting mixture can be up to 35% by volume in the process of the present invention. It is advantageously at least 20% by volume or at least 25% by volume. Reaction gas starting mixtures which are useful according to the present invention comprise xe2x89xa765% by volume and xe2x89xa685% by volume of propane plus xe2x89xa715% by volume and xe2x89xa635% by volume of molecular oxygen. According to the invention, it is advantageous (in respect of selectivity and conversion) for the molar ratio of propane to molecular oxygen in the reaction gas starting mixture to be  less than 5:1, preferably xe2x89xa64.75:1, better xe2x89xa64.5:1 and particularly preferably xe2x89xa64:1. As a rule, the abovementioned ratio will be xe2x89xa71:1 or xe2x89xa72:1.
In principle, active compositions I which are suitable according to the present invention can be readily prepared by making up an intimate, preferably finely divided dry mixture of suitable sources of their elemental constituents which has a composition corresponding to their stoichiometry and calcining this mixture at from 450 to 1000xc2x0 C. The calcination can be carried out either under inert gas or in an oxidizing atmosphere such as air (mixture of inert gas and oxygen) or else in a reducing atmosphere (e.g. mixture of inert gas, oxygen and NH3, CO and/or H2). The calcination time can be from a few minutes to a few hours and usually decreases with increasing temperature. Suitable sources of the elemental constituents of the active multimetal oxide compositions I are compounds which are already oxides and/or compounds which can be converted into oxides by heating, at least in the presence of oxygen.
Apart from the oxides, suitable starting compounds are, in particular, halides, nitrates, formates, oxalates, citrates, acetates, carbonates, amine complexes, ammonium salts and/or hydroxides (compounds such as NH4OH, (NH4)2CO3, NH4NO3, NH4CHO2, CH3COOH, NH4CH3CO2 and/or ammonium oxalate, which dissociate and/or can be decomposed at the latest during the subsequent calcination to form compounds which are all given off in gaseous form, can be additionally incorporated into the intimate dry mixture). The intimate mixing of the starting compounds to produce multimetal oxide compositions I can be carried out in dry or wet form. If it is carried out in dry form, the starting compounds are advantageously used as finely divided powders and are subjected to calcination after mixing and, if desired, compaction. However, the intimate mixing is preferably carried out in wet form. Here, the starting compounds are usually mixed with one another in the form of an aqueous solution and/or suspension. Particularly intimate dry mixtures are obtained in the mixing process described if all the sources of the elemental constituents used are present in dissolved form. The solvent used is preferably water. The resulting aqueous composition is subsequently dried, with the drying process preferably being carried out by spray drying of the aqueous mixture at outlet temperatures of from 100 to 150xc2x0 C. Particularly suitable starting compounds of Mo, V, W and Nb are their oxo compounds (molybdates, vanadates, tungstates and niobates) or the acids derived from these. This applies particularly to the corresponding ammonium compounds (ammonium molybdate, ammonium vanadate, ammonium tungstate).
In the process of the present invention, the multimetal oxide compositions I can be used either in powder form or after shaping to give particular catalyst geometries; shaping can be carried out before or after the subsequent calcination. For example, unsupported catalysts can be produced from the powder form of the active composition or its uncalcined precursor composition by compaction to give the desired catalyst geometry (e.g. by tableting or extrusion). If desired, auxiliaries such as graphite or stearic acid as lubricants and/or shaping aids and reinforcing materials such as microfibers of glass, asbestos, silicon carbide or potassium titanate can be added. Suitable unsupported catalyst geometries are, for example, solid cylinders or hollow cylinders having an external diameter and a length of from 2 to 10 mm. In the case of the hollow cylinders, a wall thickness of from 1 to 3 mm is advantageous. Of course, the unsupported catalyst can also have a spherical geometry, in which case the sphere diameter can be from 2 to 10 mm.
Of course, the shaping of the pulverulent active composition or its pulverulent but not yet calcined precursor composition can also be achieved by application to preshaped inert catalyst supports. The coating of the support bodies to produce the coated catalysts is generally carried out in a suitable rotatable container, as is known, for example, from DE-A 2909671 or EP-A 293859. To coat the support bodies, it is advantageous to moisten the powder composition to be applied and to dry it again after application, e.g. by means of hot air. The thickness of the layer of powder composition applied to the support body is advantageously selected so as to be in the range from 50 to 500 xcexcm, preferably in the range from 150 to 250 xcexcm.
Support materials which can be used here are customary porous or nonporous aluminum oxides, silicon dioxide, thorium dioxide, zirconium dioxide, silicon carbide or silicates such as magnesium silicate or aluminum silicate. The support bodies can have regular or irregular shapes, with preference being given to support bodies having a regular shape and a distinct surface roughness, e.g. spheres or hollow cylinders. Essentially nonporous, spherical steatite supports which have a rough surface and a diameter of from 1 to 8 mm, preferably from 4 to 5 mm, are useful.
As regards the preparation of the multimetal oxide compositions II, what has been said for the multimetal oxide compositions I applies. However, the calcination temperature employed is generally from 350 to 650xc2x0 C. Particularly preferred multimetal oxide compositions II are the multimetal oxide compositions I disclosed in EP-B 575897, particularly their preferred variants. In these, multimetal oxides are first preformed from portions of the elemental constituents and are used as element source in the further course of the preparation.
The process of the present invention is advantageously carried out in multitube reactors as are described, for example, in EP-A 700893 and EP-A 700714. The fixed-bed catalyst to be used according to the present invention is located in the metal tubes (in general of stainless steel) and a heat transfer medium, in general a salt melt, is passed around the metal tubes. In the simplest case, the two catalyst beds A and B to be used according to the present invention are arranged in direct succession in each reaction tube. The ratio of the bed volumes of the two catalyst beds A and B is, according to the present invention, advantageously from 1:10 to 10:1, preferably from 1:5 to 5:1 and particularly preferably from 1:2 to 2:1, frequently 1:1. The reaction pressure is generally xe2x89xa70.5 bar. Normally, the reaction pressure will not exceed 100 bar, i.e. it will be from xe2x89xa70.5 to 100 bar. It is frequently advantageous for the reaction pressure to be from  greater than 1 to 50 bar or from  greater than 1 to bar. The reaction pressure is preferably xe2x89xa71.25 or xe2x89xa71.5 bar or xe2x89xa71.75 or xe2x89xa72 bar. Frequently, the upper limit of 10 or 20 bar is not exceeded here. Of course, the reaction pressure can also be 1 bar (the above statements regarding the reaction pressure apply quite generally to the process of the present invention). Furthermore, the space velocity is advantageously selected such that the residence time of the reaction gas mixture over the two catalyst beds A and B is from 0.5 to 20 sec, preferably from 1 to 10 sec, particularly preferably from 1 to 4 sec and frequently from 3 sec. Propane and/or propene present in the product mixture from the process of the present invention can be separated off and returned to the gas-phase oxidation according to the present invention. Furthermore, the process of the present invention can be followed by a further heterogeneously catalyzed oxidation stage as is known for the heterogeneously catalyzed gas-phase oxidation of acrolein to acrylic acid, into which the product mixture of the process of the present invention, if desired with the addition of further molecular oxygen, is conveyed. At the end of this, unreacted propane, propene and/or acrolein can again be separated off and returned to the gas-phase oxidation. The acrolein and/or acrylic acid formed can be separated from the product gas mixtures in a manner known per se. In general, the propane conversion achieved using the process of the present invention is xe2x89xa75 mol %, or xe2x89xa77.5 mol %. However, propane conversions of xe2x89xa720 mol % are not normally achieved. The process of the present invention is particularly suitable for continuous operation. If necessary, additional molecular oxygen can be. injected at the level of the catalyst bed B. Conversion, selectivity and residence time referred to in this document are, unless indicated otherwise, defined as follows:             Conversion of propane (mol %)        =                            Number of moles of propane reacted                          Number of moles of propane fed in                    xc3x97      100                  Selectivity S of acrolein and/or
acrylic acid formation (mol %)        =                            Number of moles of propane converted into acrolein and/or acrlic acid                          Number of moles of propane reacted                    xc3x97      100                  Residence time (sec)        =                            free volume of the reactor (1) in the zone where the catalyst is located                          amount of reaction gas starting mixture passed through                    xc3x97      3600      